Horton

Re: POC: enable logical decoding when wal_level = 'replica' without a server restart

shveta malik <shveta.malik@gmail.com>

View thread
From: shveta malik <shveta.malik@gmail.com>
To: Masahiko Sawada <sawada.mshk@gmail.com>
Cc: Shlok Kyal <shlok.kyal.oss@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Amit Kapila <amit.kapila16@gmail.com>, Bertrand Drouvot <bertranddrouvot.pg@gmail.com>, Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>, shveta malik <shveta.malik@gmail.com>
Date: 2025-08-26T09:32:40Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Fix regression test failure when wal_level is set to minimal.

  2. Toggle logical decoding dynamically based on logical slot presence.

  3. Disallow server start with sync_replication_slots = on and wal_level < logical.

On Tue, Aug 26, 2025 at 12:54 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> I've attached the updated patch that incorporated the comments and is
> rebased to the current HEAD.
>

Thanks for the patch, please find a few comments concerning LOG messages:

1)
slotsync worker gives LOG:
LOG:  replication slot synchronization requires logical decoding to be enabled

By this LOG message, the user might not know how to enable logical
decoding. Shall we add HINT/DETAIL similar to other places:
To enable logical decoding on standby, set "wal_level" >= "logical" or
create at least one logical slot on the primary server.

2)
When we try to create a logical slot on standby, it takes some time
until runnign-txns are logged on primary. During that wait-time, if we
drop logical slot on primary disabling logical_deocding on standby,
then slot-creation fails with:

postgres=# SELECT pg_create_logical_replication_slot('st_slot2',
'pgoutput', false, false, false);
ERROR:  canceling statement due to conflict with recovery
DETAIL:  User was using a logical replication slot that must be invalidated.

Do we need to tweak the message a little bit as this new case is is
not the case of invalidation?

3)
When slot is invalidated on standby, we get message:

LOG:  invalidating obsolete replication slot "st_slot"
DETAIL:  Logical decoding on standby requires "wal_level" >= "logical"
or to create at least one logical slot on the primary server.

The DETAIL msg looks slightly odd. Shall we make it as:
Logical decoding on standby requires "wal_level" >= "logical" or at
least one logical slot on the primary server.

thanks
Shveta